The FBI warned U.S. businesses that hackers have used malicious software to launch destructive attacks in the United States, following a devastating cyberattack last week at Sony Pictures Entertainment.

The five-page, confidential "flash" warning issued to businesses late on Monday provided some technical details about the malicious software that was used in the attack, though it did not name the victim.

An FBI spokesman declined comment when asked if the software had been used against the California-based unit of Sony Corp.

An FBI official told NBC News that the advisory gives information security personnel details about the Sony attack, including several pages of computer coding. The advisory is not about a new malware attack.

Officials are still analyzing the Sony attack, which caused serious damage to that company's databases, according to NBC News.

The Sony attack resulted in five films being leaked online, including the updated version of "Annie." In the attack on the studio's corporate systems Nov. 24, an image of a skeleton appeared on company computers with a message that said, "Hacked by #GOP," with the group behind it calling itself "Guardians of Peace."

The message threatened to release "secrets and top secrets" of the company. Currently being investigated is a connection between upcoming Sony movie "The Interview," and North Korea.

The FBI occasionally issues "flash" warnings to provide businesses with details about emerging cyber threats to help them defend against new types of attacks. It does not name the victims of those attacks in those reports.

The report said that the malware overrides data on hard drives of computers which can make them inoperable and shut down networks.

It is extremely difficult and costly, if not impossible, to recover hard drives that have been attacked with the malware, according to the report, which was distributed to security professionals at U.S.